Essential equipment
Battery pack if you get her demanding games like Zelda and Mario Kart.
Here is the average battery lifespan for many games.
More stats in the following link
https://m.imgur.com/a/aODDLei
Screen protector with glare resistence.
Switch Even in partial shade can be a huge disappointment. You already need the screen protector to prevent the dock from scratching it.
Atleast another left joycon.
There is a known manufacturing defect with Joycons and it is more common with the left side. It is ridiculous that Nintendo asks you to send back the entire console only to end up replacing the defective Joycon with a new one.
That is your essential equipment.
As for games I recommend you get your kid the following as my top 7 recommendations
puzzle / platformer
Toki Tori +
Match 3
Choose 2 of these 3
Lumines, Battle Chef Brigrade, Puyo Puyo Tetris
Collectible RPG to satiate that Pokemon fix
Nothing really. The closest to it is
Darkest Dungeon but a game like that requires discretion on your part. It could be handled by a 15 old. But a 9 year old requires a lot of guidance to deal with its themes.
As for other downloadables:
Fallout Shelter
Respectable amount of content and can easily be played for free. It's a lot of fun.
West of Loathing
Comedy games are rare and comedy in general that anyone can laugh with is even rarer across all entertainment media.
Hollow Knight
This is the best game you can get at its price point. It rivals even some of Nintendo's $60 msrp games. Don't ignore it.
Human Fall Flat
This game was not for me but the family and friends play factor is undeniable. As a single player game it does hold up better than overcooked.
Shovel Knight
Best platformer you can buy. In certain ways it is better than Odyssey.
While you are focused on downloadables I also recommend these bigger titles.
Fire Emblem Warriors
Very easy to get into and the difficulty scaling can be controlled. Hard to find actually good real time strategy or tactical games on a console but this is one of them.
Mario Tennis Aces
Simply inform her that star power is gained and blocked by swinging back with an opposing color and she will figure out all the other tricks and strategies on her own. Great multiplayer game.
BOTW
For obvious reasons
Xenoblade 2
I never got this because of they performance but it still looks like a good game and is remotely the only other game that could match her Pokemon fix :)
Price drop won't happen but expect a ton of sales or bundles for the holidays.
There have already been a ton of great deals to save as much as $80 on Switch this year.